Conventionally, a chemical vapor deposition (CVD) method for a polycrystal silicon layer has been used in manufacturing a metal-insulator-semiconductor field effect transistor (MISFET), for example, a MIS thin film transistor (MISTFT), in which a polycrystal silicon layer formed on a substrate is used as source, drain and channel regions.
In the case of forming a polycrystal silicon layer of this type by the ordinary CVD method, reactive species which are produced by decomposition of a material gas in a vapor phase reach the substrate and react on the substrate, thereby forming a film. Alternatively, the reactive species react in a region very close to the surface of the substrate and are deposited thereon. In order for the film to be produced and epitaxially grow, the reactive species must migrate on the surface of the substrate.
In a plasma CVD method known as a CVD method, a two-frequency method for utilizing plasma potential control with the action of a high-frequency field or for applying a low-frequency bias field is used to control the migration or the kinetic energy of deposition species. In an ion cluster beam (ICB) method, an acceleration voltage is controlled.
These film forming methods have problems as follows.
First, in the case of the plasma CVD method, the use of plasma leads to the following drawbacks.
(1) Lack of uniformity and fluctuation of a plasma field, and a non-uniform electric field in plasma-induced electric charges are generated. These may cause damages and short circuits t the transistor (e.g., charge-up or discharge breakdown of a gate oxide film, discharge between wirings, and the like). Particularly, such phenomenon tends to occur at the time of switching on/off the plasma.
(2) There is a possibility of ultraviolet damage due to light emission from the plasma.
(3) Plasma discharge is difficult in a large area, and occurrence of a standing wave makes it difficult to realize uniformity.
(4) The device is complicated and expensive and requires complicated maintenance work.
In the case of the ICB method, too, since cluster ions are led onto the substrate through an aperture of an accelerating electrode so as to collide with the substrate, it is difficult to realize uniformity and to form a film of a large area, that is, a film on a large substrate.
On the other hand, the catalyzed CVD method disclosed in the Japanese Publication of Unexamined Patent Application No. S63-40314 draws attention as an excellent CVD method which enables formation of a polycrystal silicon film or a silicon nitride film at a low temperature on an insulating substrate such as a glass substrate.
According to the catalyzed CVD method, for example, a silane gas is brought in contact with a heated metal catalyzer and is thus decomposed, thereby forming reactive species having high energy, for example, a radical silicon molecule or a group of molecules, a silicon atom or a group of atoms, and a radical hydrogen ion. These are brought in contact with the substrate so as to react and be deposited thereon. Therefore, a silicon film can be deposited in a large area at a temperature lower than the deposition temperature of the ordinary thermal CVD method and without using plasma.
In the catalyzed CVD method as described above, formation of a film is controlled by a relatively small number of parameters such as the temperature of the substrate, the temperature of the catalyzer, the gas pressure or the flow rate of the reaction gas. Although this proves that the catalyzed CVD method is a simple method, particularly the momentum of deposition species can only be controlled in accordance with the kinetic theory of gases. That is, the migration or the kinetic energy of deposition species is only the thermal energy in vacuum. Since it depends exclusively on the thermal energy, lowering of the deposition temperature is restricted. Therefore, it is difficult to use a plastic film substrate having a poor heat resistance property and the degree of freedom in selection of the substrate material is limited. Also, since the control of the momentum of deposition species is insufficient, burying of a metal for connection into a via-hole (through-hole for connection between wirings) having a particularly large aspect ratio and the step coverage tend to be insufficient.